In eukaryotic protein synthesis, all three termination codons are directly recognized by a polypeptide-chain releasing factor, eRF1,to release polypeptide chain from ribosome, and another releasing factor, eRF3,is required for eRF1 binding to ribosomal A site. We previously reported that a GTP-binding protein, whose carboxy-terminal sequence is homologous to the eukaryotic elongation factor EF1α, forms a complex with eRF1 to function as eRF3 in the translation termination and that eRF3 consists of the EF1α-like carboxy-terminal (C) and amino-terminal (N) domains. In the present studies, we investigated the structures and functions of eRF3 and its related G proteins. The major findings obtained in this study are summarized as follows.1)The C domain of eRF3 associated with eRF1,whereas the N domain was capable of binding to poly-adenylate binding protein (PABP) associating with the poly(A) tail of mRNAs presumably for their stabilization. 2)When the interaction between eRF3-N domain and PABP was abolished, decay rate of all mRNAs was decreased due to the inhibition of poly(A) tail shortening. 3)We identified a poly(A) nuclease (PAN) complex that is activated by PABP and found that eRF3 and PAN complex competitively bound to PABP. These results indicate that the translation termination-coupled mRNA decay is mediated through eRF3-dependent poly(A) shortening. Thus, eRF3 functions not only as an eRF1-carrier protein in the translation termination but also as an initiator of the mRNA degradation machinery. 4)We also investigated the functions of other G proteins (Ski7 and eRFS) structurally related to eRF3 and found that the N domains of these G proteins commonly interact with factors involved in mRNA-degradation machineries to regulate mRNA stabilization.